This invention relates to servo systems for disc drives, and particularly to servo systems employing thermal signals.
Magnetoresistive (MR) heads operate on the principle that the resistance of a magnetoresistive element is at least in part based on the strength of the magnetic field from the data medium adjacent the head. A bias current through the MR element creates a voltage across the head, the magnitude of the voltage being based on the magnitude of the bias current and the resistance of the MR element. The varying voltage signal is representative of the data on the disc.
The resistance of the MR element is also dependent on a number of other factors, including temperature. Thus, the bias current also generates about 10 to 100 mW power in the form of heat within the element. The heat thermally dissipates into the head as well as across the interface into the recording disc. The amount of heat dissipated across the interface into the disc is a function of disc spacing between the disc and MR element. At small head/disc spacings (low flying heights), more heat is dissipated to the disc than when the spacing is large (high flying heights). Hence, the resistance of the MR element is a function of magnetic field strength, as well as thermal dissipation due to a function of disc/head spacing.
Typically, data are recorded on concentric tracks of magnetic discs, and the MR head is positioned over the selected track by an actuator. Servo data are used to control the actuator to accurately position the head over the selected track, and to maintain the head on the track. Usually, servo data are recorded either in the form of servo bursts in "spokes" on the same disc containing the user data (embedded servo systems) or on a separate disc maintained for the purpose of servo control (dedicated servo system). In both the dedicated and embedded servo systems, areas of the disc surface are dedicated to the recording of servo data, either on the same disc as user data or on the separate servo disc. The area dedicated to servo data may encompass a significant portion of the total area of the discs available for data of all classes. More particularly, in a dedicated servo system employing six discs (twelve disc surfaces) with one of the surfaces dedicated to the servo data, more than 8% of the total area of all disc surfaces is dedicated to servo data. In an embedded servo system, servo data may comprise 8 to 15% of the length of a track, and as much as 12% of the entire area of a disc. Because the servo data represents such a significant area of the disc, it is desirable to reduce that area without adversely affecting the operation of the disc drive.
Servo data are ordinarily recorded at significant lower frequencies than user data. For example, while user data may be in the MegaHertz range (e.g., 20 MHz and higher), servo data is ordinarily in the KiloHertz range (typically below about 20 KHz). While it would seemingly be possible to modulate the low frequency servo data with the high frequency user data on the magnetic disc, in fact it has not been practical. More particularly, data (both servo and user data) are recorded as transitions between magnetic states, the states generating magnetic fields of a single strength. The writing of magnetic states associated with the user data would overwrite the previously written magnetic states associated with the servo data, thereby rendering impossible recovery of servo data. Even if it were possible to preserve the transitions associated with the servo data, they would be indistinguishable from user data transitions, resulting in imprecise detection of the locations of transitions between magnetic states on the recording disc, thus leading to errors in user data recovery.